Carbonyl sulfide (COS) is an undesirable impurity in materials such as petroleum hydrocarbons because it is a source of sulfur, and therefore a potential atmospheric pollutant. COS also acts as an undesirable contaminant of industrial processes by poisoning polymerization catalysts when present in petroleum-derived polymerizable olefins, such as propylene. COS may be present in such processes as a contaminant initially present in the feedstock, or it may be formed in a treating process, such as being the result of the molecular sieve-catalyzed reaction of carbon dioxide with hydrogen sulfide or other sulfur compounds.
Prior art methods of removing COS can be divided into three categories: distillation, hydrolysis, and the use of adsorbents. Each of these methods has certain disadvantages.
U.S. Pat. No. 3,315,003 discloses a process for removing COS from a hydrocarbon by first contacting the hydrocarbon with a liquid such as monoethanolamine which scrubs the hydrocarbon to remove acid gases such as H2S and CO2 and part of the COS. The hydrocarbon is then distilled. After several subsequent distillations, the liquid bottom product is treated with a soda-lime to remove any remaining COS. However, distillation processes are extremely inefficient due to the cost of energy to vaporize virtually all of the liquid.
U.S. Pat. No. 3,265,757 teaches the catalytic hydrolysis of COS to form H2S, using alumina as a catalyst. A mixture of the liquid hydrocarbon and water is contacted with a high surface area alkali-impregnated, active alumina containing from 0.15 to 3 wt-% of sodium or potassium at a temperature of from 20° to 50° C. The patent states that the hydrolysis reaction will not commence if the alumina is bone dry, and suggests either moistening the alumina catalyst with ion-free water prior to the reaction, or passing a mixture of ion-free water and the liquid hydrocarbon through the catalyst bed until a sufficient amount of water has built up on the alumina to permit the hydrolysis reaction to proceed. However, while this process does remove COS (by converting it to H2S), it does not remove sulfur per se from the hydrocarbon, but merely changes the form of the sulfur compound which still must be subsequently removed from the hydrocarbon by another process step.
U.S. Pat. No. 4,455,446 teaches the removal of COS from propylene by hydrolysis over a catalyst comprising platinum sulfide on alumina. The patent states that the hydrolysis reaction may be carried out in either the gaseous or liquid phase with a temperature of 35° to 65° C. used for the liquid phase. An amount of water at least double the stoichiometric amount of the COS to be hydrolyzed must also be present.
The disadvantages of these prior art methods of removing COS include the requirement that the stream be preconditioned with water and that there be a subsequent treatment to remove both the hydrolysis products and the water. In addition, the residual COS content in the effluent may still be too high, especially in view of the requirements of the particular polymerization process downstream.
U.S. Pat. No. 4,835,338 describes a process for the removal of sulfur impurities from liquid hydrocarbons in which an activated alumina adsorbent is used to remove the COS from a liquid propylene stream. In this process, the absorbent is regenerated by passing a heated gas through the adsorbent. The disadvantage of this process is that after a few cycles, typically four to six regeneration cycles, the adsorbent COS capacity decreases in each successive cycle until it stabilizes at a level of about 40% of fresh equilibrium capacity. This low level of regeneration of the adsorbent means that a significantly higher quantity of adsorbent is required in order to achieve the desired removal of COS than would be necessary if complete regeneration of the adsorbent bed was achieved after each cycle.
U.S. Pat. No. 6,843,907, which is incorporated herein by reference, describes an improved adsorbent process. The process involves contacting a hydrocarbon stream containing COS with an adsorbent and then regenerating the adsorbent by passing a heated gas containing a hydrolyzing agent through the absorbent. The heated gas is generally heated to a temperature of from about 100° to 350° C. The adsorbent that is regenerated by using this process retains at least 70% of its capacity for adsorption of sulfur as compared to fresh adsorbent.
There remains a need for improved methods for removal of COS from hydrocarbon streams.